Method of treating an article with a plasma apparatus in which a uniform electric field is induced by a dielectric window

ABSTRACT

A method of plasma treating an article in a housing having a chamber in which the article such as a wafer can be treated with plasma. The housing includes at least one inlet port connected to an interior of the chamber through which process gas can be supplied to the chamber. A radiofrequency energy source is arranged to pass radiofrequency energy into the chamber and induce plasma in the interior of the chamber by activating, with an electric field induced by the radiofrequency energy source, process gas supplied to the chamber through the inlet port. A dielectric window having an inner surface thereof forms part of an inner wall of the chamber. Radiofrequency energy passes from the radiofrequency energy source to the interior of the chamber through the dielectric window. The dielectric window has a thickness which varies at different points along the inner surface thereof such that the thickness is largest at a central portion of the dielectric window. The dielectric window is effective to decrease the induced electric field in the interior of the chamber near the central portion of the dielectric window.

This application is a divisional of application Ser. No. 07/883,201,filed May 14, 1992 now U.S. Pat. No. 5,226, 967.

BACKGROUND OF THE INVENTION

The invention relates to apparatuses for processing of substrates usingradiofrequency induced plasma in a plasma chamber. In particular, theinvention provides apparatuses and methods for generating a plasma of auniform plasma density.

Gaseous plasma technology is a well known technique used for thefabrication of integrated circuits. Parallel plate reactors have beenused extensively for exciting the gases in the reaction chamber togenerate the chemical reactions required for thin film etching anddeposition of wafers. In general, when coupling power through aninsulator, previous hardware setups have used 13.56 MHz as the excitingfrequency for the gases due to a higher excitation efficiency. Forinstance, see U.S. Pat. No. 4,948,458 ("Ogle"), the disclosure of whichis hereby incorporated by reference.

In apparatuses such as that shown by Ogle, a radiofrequency magneticfield is induced in a low pressure reaction chamber by sending aradiofrequency resonant current through an external planar coil andpassing the generated radiofrequency energy through a dielectric windowin the chamber. The magnetic field generates a plasma by causing acirculating flux of electrons in a process gas introduced into thechamber to produce a region of ionic and radical species. The plasma sogenerated is used to etch or deposit materials on a wafer in thechamber.

It has been found that the plasma density across the surface area of thewafer is highly variable in such apparatuses, with densities measuredacross 150 mm and 200 mm wafer areas being as much as two times as greatin some areas than in others. This non-uniform plasma density causessignificantly non-uniform oxide and resist etch rates over measuredwafer areas and makes it extremely difficult to control criticaldimensions of fine line geometry on the wafer.

Normally, a flat dielectric window is used with the apparatuses. It hasbeen observed that the magnetic flux of the planar coil is highest nearthe window center and, with a flat window, the induced electric field isconsequently higher near the window center. The apparatuses and methodsof the present invention utilize a dielectric window having acharacteristic cross section, wherein the window is thicker at thecenter and thinner at the edges, to decrease the higher induced electricfield near the window center.

SUMMARY OF THE INVENTION

An apparatus according to one aspect of the present invention includes ahousing having a chamber in which a semiconductor wafer can be treatedwith plasma, the housing including at least one inlet port connected toan interior of the chamber through which process gas can be supplied tothe chamber. The apparatus further includes a radiofrequency energysource that is arranged so as to pass radiofrequency energy into thechamber and induce plasma in the interior of the chamber by activating,with an electric field induced by the radiofrequency energy source,process gas supplied to the chamber through the inlet port. A dielectricwindow having an inner surface thereof forming part of an inner wall ofthe chamber is arranged such that radiofrequency energy from theradiofrequency energy source can be passed to the interior of thechamber through the dielectric window. The dielectric window has athickness which varies at different points along the inner surfacethereof such that the thickness is largest at a central portion of thedielectric window, the dielectric window being effective to decrease theinduced electric field in the interior of the chamber near the centralportion of the dielectric window.

The radiofrequency energy source can comprise a substantially planarplasma generating electrode having one planar face thereof facing anouter planar surface of the dielectric window. The dielectric window canbe circular in shape. The dielectric window can comprise a plurality oflayers of the same or different dielectric materials. The dielectricwindow can also include at least one step therein such that thedielectric window has a region of reduced thickness surrounding thecentral portion. The dielectric window can include at least one taperedsurface surrounding the central portion or the dielectric window can beconvex in shape.

The invention also provides a method for treating an article with plasmacomprising steps of placing an article within a chamber and introducingprocess gas into the chamber and generating a uniform electric field inthe chamber by passing radiofrequency energy through a dielectric windowin the chamber. The dielectric window has a thickness which varies atdifferent points along an inner surface thereof such that the thicknessis largest at a central portion of the dielectric window. As a result,the uniform electric field generates a uniform electron flow in theprocess gas and thereby generates a plasma of uniform plasma density.The process further includes the step of plasma treating an article byexposing a surface of the article to the plasma generated in thechamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be wellunderstood by reading the following detailed description in conjunctionwith the drawings in which like numerals indicate similar elements andin which:

FIG. 1 is an isometric view of an apparatus for producing a planarplasma in accordance with the present invention;

FIG. 2 is a cross-sectional view of the apparatus of FIG. 1;

FIG. 3 is a schematic view of the circuitry of the apparatus in FIG. 1;

FIG. 4 is a schematic view illustrating the magnetic field profileproduced by the apparatus of FIG. 1;

FIG. 5 is a graphic representation of ion current density versusdistance from a center of a wafer in an apparatus having a dielectricwindow with a flat cross-section;

FIG. 6 is a graphic representation of ion current density versusdistance from a center of a wafer in an apparatus according to thepresent invention;

FIG. 7 is a side view of an embodiment of a dielectric window accordingto the present invention;

FIG. 8 is a side view of another embodiment of a dielectric windowaccording to the present invention; and

FIG. 9 is a side view of a further embodiment of a dielectric windowaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, apparatus and methods are providedfor producing highly uniform, planar plasmas over relatively largeareas. The ionic and radical species produced in the plasma experienceminimum acceleration in non-planar directions, and the resulting plasmathus has very low desired kinetic energy. In addition, uniform planarplasma can be produced over very wide pressure ranges, typically from10⁻⁵ Torr to 5 Torr, and greater.

The apparatus of the present invention comprises a housing having aninterior chamber bounded at least in part by a dielectric window. Aplanar coil is disposed proximate the window, and a radiofrequencysource is coupled to the coil. Usually, the radiofrequency source iscoupled through an impedance matching circuit to maximize power transferand a tuning circuit to provide for resonance at the operatingfrequency, typically 13.56 MHz. Inlet ports are provided for supplying aprocess gas to the chamber. By resonating a radiofrequency currentthrough the coil, an electromagnetic field is induced which extends intothe chamber through the dielectric window. In this way, a flow ofelectrons may be induced. Moreover, as the electrons are closelyconfined to a planar parallel to the planar coil, transfer of kineticenergy in non-planar directions is minimized.

The chamber includes a support surface for a planar article, typically asemiconductor wafer. The surface supports the wafer in a plane which isparallel to the plane of the coil, and hence, also parallel to the planeof the plasma. Thus, the semiconductor wafer is exposed to a uniformplasma flux, ensuring uniform plasma treatment. As the plasma specieshave minimum kinetic velocities in non-planar directions, their kineticimpact on the wafer is minimized. Thus, the treatment can be generallylimited to the chemical interaction of the plasma species with thewafer.

A velocity component in the direction normal to the surface of thesemiconductor wafer may be provided by applying a radiofrequencypotential in a direction normal to the plane of the plasma.Conveniently, such a potential may be applied by the support surfaceupon which the wafer is maintained. For instance, the support surfacecan include a conventional bottom electrode for supplying such apotential.

The method and apparatus of the present invention are useful in avariety of semiconductor processing operations, including plasma etchingsuch as etching of an aluminum layer on a semiconductor substrate,deposition processes, resist stripping, plasma enhanced chemical vapordepositions, and the like.

The housing defines a generally air-tight interior chamber wherein theplanar plasma is to be generated. The housing includes at least oneinlet port for introducing a process gas and at least one outlet portfor connection to a vacuum system for maintaining a desired operationpressure within the chamber. Systems for supplying a preselected processgas and for maintaining a preselected pressure within the chamber arewell known in the art and need not be described further. One or moresurfaces within the chamber support the articles to be treated.Typically, the surfaces will be disposed in a preselected orientationrelative to the planar plasma which is to be generated within thechamber, usually being generally parallel to the plane of the plasma.

In order to induce the desired planar plasma, an electrically-conductivecoil is disposed adjacent to the exterior of the dielectric window. Thecoil is substantially planar and generally comprises a single conductiveelement formed into a planar spiral or a series of concentric rings. Byinducing a radiofrequency current within the coil, an electromagneticfield is produced which will induce a flow of electrons within a planarregion parallel to the plane of the coil.

The planar coil is generally circular, although ellipsoidal patterns andother deviations from true circularity can be tolerated. Moreover, thecoil may be truly planar across its diameter, or may deviate somewhatfrom true planarity. Deviations from planarity can be less than 0.2 ofthe diameter of the coil, usually being less than 0.1 of the diameter.Adjustments to the profile of the coil may be made to modify the shapeof the field which is generated. The diameter of the coil will generallycorrespond to the size of the plasma which is to be generated. Coildiameters may range from about 8 cm to 30 cm, usually from about 13 cmto 18 cm. For the treatment of individual semiconductor wafers, the coildiameter will generally be from about 13 to 18 cm.

The coil includes a sufficient number of turns in order to produce arelatively uniform magnetic field across its entire diameter. The numberof turns will also depend on the diameter of the coil. A coil sized fortreating individual semiconductor wafers usually has from abut 5 to 8turns. The resulting inductance of the coil will usually be from 1.2 to3.5 μH, with an impedance in the range from about 20 to 300 Ohms.

Conveniently, the planar coil may be formed from any electricallyconductive metals, usually being formed from copper. The coil can have aload carrying capacity in the range from abut 5 to 100 amps.

The planar coil is disposed next to a dielectric window forming part ofthe treatment chamber. The dielectric window maintains the isolation ofthe interior of the chamber, while allowing penetration of the magneticfield produced by the planar coil. The remainder of the housing can bemetal. The dielectric window can be composed of quartz, although otherdielectric materials, particularly ceramics which do not absorb energyat the frequency of operation, may be used. Conveniently, a dielectricwindow may be placed adjacent to a port formed in a wall of the housing.The geometry of the port usually corresponds to that of the planar coil,typically being circular. The planar coil can be disposed very close toor touching the dielectric window in order to maximize the intensity ofthe magnetic field produced within the chamber. The thickness of thedielectric window is thin enough to transmit the energy to the plasma,usually being selected to be sufficient to withstand the differentialpressure created by the vacuum within the chamber. For example, thewindow can be at least one-half inch thick or thicker.

The planar coil is driven by a radiofrequency (RF) generator of a typewhich is generally utilized in the operation of semiconductor processingequipment. The RF generator will usually operate at a frequency in therange from about 13.56 MHz to 100 MHz, typically being operated at 13.56MHz. The RF generator usually has a low impedance, typically about 50Ohms, and will be capable of producing from about 1 to 6 amps, usuallyfrom about 2 to 3.5 amps, with an RMS voltage of at least about 50volts, usually being at least about 70 volts, or more. Conveniently, theRF generator can have an output connector in the form of a coaxial cablewhich may be connected directly to the circuitry operating the planarcoil.

Referring to FIGS. 1 and 2, a plasma treatment system 10 suitable foretching individual semiconductor wafers W includes a chamber 12 havingan access port 14 formed in an upper wall 16. A dielectric shield/window18 is disposed below the upper wall 16 and extends across the accessport 14. The dielectric window 18 is sealed to the wall 16 to define avacuum-tight interior chamber 19 of the chamber 12.

A planar coil 20 is disposed within the access port 14 adjacent todielectric window 18. Coil 20 is formed as a spiral having a center tap22 and an outer tap 24. The plane of the coil 20 is oriented parallel toboth the dielectric window 18 and a support surface 13 upon which thewafer W is mounted. In this way, the coil 20 is able to produce a planarplasma within the chamber 19 of the chamber 12 which is parallel to thewafer W. The distance between the coil 20 and the support surface 13 canbe in the range from about 3 to 15 cm, more usually from about 5 to 20cm with the exact distance depending on the particular application.

Referring now to FIGS. 1-3, the planar coil 20 is driven by an RFgenerator 30 of the type described above. The output of the generator 30is fed by a coaxial cable 32 to a matching circuit 34. The matchingcircuit 34 includes a primary coil 36 and a secondary loop 38 which maybe mutually positioned to adjust the effective coupling of the circuitand allow for loading of the circuit at the frequency of operation.Conveniently, the primary coil 36 is mounted on a disk 40 which may berotated about a vertical axis 42 in order to adjust the coupling.

A variable capacitor 44 is also provided in series with the secondaryloop 38 in order to adjust the circuit resonant frequency with thefrequency output of the RF generator 30. Impedance matching maximizesthe efficiency of power transfer to the planar coil 20. An additionalvariable capacitor 46 can be provided in the primary circuit in order tocancel part of the inductive reactance of coil 36 in the circuit.However, other circuit designs may also be provided for resonantlytuning the operation of planar coil 20 and for matching the impedance ofthe coil circuit with the RF generator.

FIG. 4 shows a desired magnetic field profile 60 in a conventionalapparatus using a flat dielectric window 18 and a planar coil 20. At theedges of the coil 20, the magnetic field strength is less than at thecenter. The induced magnetic field causes a circulating flux ofelectrons in the plasma created by collision of the electrons with theindividual molecules of the process gas, and in turn produces a regionof ionic and radical species. FIG. 5, however, shows that, with such anapparatus having a window of uniform thickness, ion current densitydrops off sharply as ion current density is measured farther and fartherfrom the center position 0 of the window 18. The dotted line in FIG. 5corresponds to the outer edge of a 6 inch wafer.

If the wafer W is small enough, then the reduced ion current density atextreme edges should not adversely affect oxide and resist etch rates onthe wafer W. However, it is common to use wafers W having sufficientlylarge diameters that the reduced ion current density at distances fromthe center of the coil 20 and window 18 does adversely affect oxide andresist etch rates on the wafer W.

Substantially uniform ion current density, usually within ±5% over theentire diameter of 150 mm and 200 mm diameter wafers, as shown by thegraph of test results set out in FIG. 6, is made possible by the presentinvention by providing a window 18 having a thickened center portion. Inparticular, the ion current density is substantially uniform from centerposition 0 to distances of at least 750 mm from the center position 0.The dotted line in FIG. 6 corresponds to an edge of an 8 inch wafer.

As is shown in FIGS. 7-9, the window 18 according to the invention canhave various cross sections. Several different types of window materialmay be used for the dielectric window 18, including ceramic, quartz orglass materials. The most advantageous window cross section under theparticular intended use conditions will be a function of the dielectricconstant of the particular window material that is chosen and powersupplied to the coil. For instance, in the case where 500 Watts issupplied to the coil, the ratio (t_(c) /t_(e)) of center thickness t_(c)to edge thickness t_(e) is about 3:1. If the power is increased to 1000Watts, the ratio t_(c) /t.sub.,e is preferably about 1.5:1. On the otherhand, if the power is lowered to 200 Watts, the ratio t_(c) /t_(e) ispreferably about 6:1.

The window 18 having a thickened center may be formed by machining ormolding a particular dielectric material such as Al₂ O₃, ZrO₂, SiO₂,etc. to form a particular lens cross section. For instance, window 18can be formed by laminating (such as by sintering) together a series ofprogressively smaller window portions 181, 182, 183 which form a seriesof steps, as shown in FIG. 7. In FIG. 7, portions 181 and 183 areone-half inch in thickness and portion 182 is one-quarter inch inthickness. The progressively smaller window portions 181, 182, 183 may,of course, also be machined or molded from a single piece of dielectricmaterial. Alternatively, window 18b can have the convex cross sectionshown in FIG. 8 or the truncated cone cross section of the window 18cshown in FIG. 9. In FIG. 9, window 18c includes a tapered surfacesurrounding a thicker central portion of the window. The window 18 canalso be made by laminating together materials having differentdielectric properties, such as ceramic materials laminated togetherpreferably without adhesive.

According to a preferred embodiment of the invention, dielectric window18 comprises a flat disc of Al₂ O₃ having a diameter of 9 to 10 inches.Such a window can be held by suitable seal means in a 12 inch diameteropening in a plasma chamber. The thickened central portion of the windowis preferably formed by a flat disc of Al₂ O₃ having a diameter of about5 to 6 inches. The two pieces of Al₂ O₃ can be laminated together bysintering and the ratio of diameters of the two discs can be about 2:1.If the coil is supplied with 500 Watts, the thickness t_(e) at the outeredge of the window is preferably 1.0 inch and the thickness t_(c) of thecenter of the window is preferably 1.5 inch.

The thickened central portion of the window 18 is ordinarily disposed onthe inside of the chamber 12, with a flat outer surface of the window 18facing outwardly from the chamber. Nonetheless, different characteristiccross sections, configurations, materials, and window thicknesses may befound to be more efficacious for particular applications.

It is, of course, possible to embody the invention in specific formsother than those described above without departing from the spirit ofthe present invention. The embodiments described above are merelyillustrative and should not be considered to be restrictive in any way.The scope of the invention is given in the appended claims, rather thanthe preceding description, and all variations and equivalents which fallwithin the range of the claims are intended to be embraced therein.

What is claimed is:
 1. A method for treating an article with plasmacomprising the steps of:placing an article within an interior of achamber; introducing a process gas into the chamber; generating auniform electric field in the chamber by passing radiofrequency energythrough a dielectric window in the chamber and inducing an electricfield in the interior of the chamber, the dielectric window having aconfiguration which reduces the induced electric field in the interiorof the chamber near a central portion of the dielectric window, theuniform electric field generating a uniform electron flow in the processgas and thereby generating a plasma of uniform plasma density; andplasma treating the article by exposing a surface of the article to theplasma generated in the chamber.
 2. The method of claim 1, wherein thearticle comprises a semiconductor substrate and the plasma treating stepcomprises etching of an aluminum layer on the semiconductor substrate.3. The method of claim 1, wherein ion current density across the articlein the chamber varies by no more than 5% across an area of at least 150mm in diameter.
 4. The method of claim 1, wherein the plasma treatingstep comprises etching an oxide on a semiconductor wafer.
 5. The methodof claim 1, wherein the plasma treating step comprises stripping aresist on a semiconductor wafer.
 6. The method of claim 1, wherein theplasma treating step comprises deposition of a layer on a semiconductorwafer.
 7. The method of claim 1, wherein the window comprises a singlepiece of quartz.
 8. The method of claim 1, wherein the window comprisesa plurality of layers of different dielectric materials.
 9. The methodof claim 1, wherein the window is thickest at the central portionthereof.
 10. The method of claim 1, wherein the window includes a flatouter surface.
 11. The method of claim 1, wherein the window issubstantially circular in shape.
 12. The method of claim 1, wherein thewindow includes a region of reduced thickness surrounding the centralportion.
 13. The method of claim 1, wherein the window is convex inshape.